Sensory processing in the auditory periphery is modified by the action of two feedback systems: the middle ear muscle (MEM) reflex and the olivocochlear (OC) reflex. Sound stimulation of the auditory nerve can lead to the excitation of "effector" neurons in both of these reflexes: i.e., MEM motoneurons from the Vth or VIIth cranial nerves and OC cells originating from the superior olivary complex. For both systems, reflex activation can lead to the suppression of responses in the auditory nerve, thus the two systems must interact functionally in the intact animal. Such an interaction may be reflected in anatomical interconnections between the pathways. The brainstem origins of these effector neurons have been well described for both systems. However, the auditory-nerve fibers project only as far central as the cochlear nucleus, and very little is known about the ascending connections from the cochlear nuclei by which these reflex loops are completed. The present project seeks to describe the wiring diagram for each of these reflex pathways and to understand the functional contribution of each component. Our approach is to exploit newly developed trans-synaptic markers in the elucidation of the anatomical pathways, and to use a combination of focal lesions and electrical excitation to study the functional contributions. The cumulative experience of the members of this research team in studying morphology and physiology in both of these reflex pathways puts us in a unique position to address these issues. Understanding these pathways is important in the clinical interpretation of MEM reflex tests and ultimately in understanding the extent to which malfunction of these systems can contribute to the difficulties of the hearing impaired.